1. Field of the Invention
This invention relates to an electrophotographic copying apparatus for exposure-scanning an original through a slit, and irradiating onto an electrified photosensitive member the reflected light from the original to thereby form on the photosensitive member an electro-static latent image, which is developed with toner, the image developed therewith being transferred to copying paper.
2. Description of the Prior Art
The electrophotographic copying apparatus electrifies the photosensitive member by applying thereto high voltage, and irradiates onto the electrified photosensitive member the reflected light from the original obtained by exposure-scanning thereof, thereby forming on the photosensitive member the electro-static latent image corresponding to the image of original. Then, the electro-static latent image is developed with the toner to thereby be sensible-imaged so that the sensible image is transferred to the copying paper.
In such electrophotographic copying apparatus, the photosensitive member has a surface area corresponding to the overall area of an original supporting plate. In a case where the original is smaller than the original supporting plate and is copied by equal magnification, unless the light is adapted to be reflected from the portion at the plate not covered by the original thereon, when the original is exposure-scanned, the charge on the side edge of photosensitive member other than the image of original is not dissipated so that the side edge is attached with the toner to be developed. Therefore, a white lid covers the original supporting plate to reflect the light even when the side edge of the original is exposed, but when the lid is open, the unnecessary attaching of toner is not avoidable.
When the charge on the abovementioned unnecessary portion is not dissipated, a very thick toner is attached onto the photosensitive member so that, when a copying paper larger than the original is copied in equal size, an unnecessary band-like image is formed around the image of original on the paper, resulting in deterioration of quality of copying. Also, even after copying, toner usually remains on the photosensitive member, which need be removed so as to prevent deterioration of the copying quality. Since the side edge portions other than the image of original on the photosensitive member are attached with the very thick toner as abovementioned, the toner is not completely transferred, but remains on the photosensitive member. For copying paper equal in size to the original, the toner at the side edge of photosensitive member is not transferred to the copying paper, but remains as a whole on the photosensitive member, whereby removal of the attached toner not only largely loads the machine but also produces much dust around the same, and further a large amount of toner is wasted so as to be not economical. For copying paper smaller than the original, the same problem as the above will be created.
In order to solve the above problem, an erase light source has been proposed which dissipates charge on the side edge areas of the photosensitive member at first electrified at the overall surface, the side edge areas being positioned perpendicularly to the moving direction of photosensitive member and unnecessary for forming the image of original.
Also, a device (disclosed in the U.S. Pat. No. 3,792,913) has been developed which detects the size of copying paper to change corresponding thereto the size of the projection areas of erase light source at the side edges perpendicular to the moving direction of photosensitive member, which dissipates the charge in the area of the photosensitive member not contacted by the copying paper during the transfer, thereby preventing unnecessary toner from attaching to the photosensitive member.
Recently, an electrophotographic copying apparatus has been developed, in which the image of original can be copied in reduction or enlargement via selective variation of the copying magnfication. The image of original, when copied in reduction, is formed on the photosensitive member in reduction by a set reduction rate relative to the actual original, resulting in that the electrified portion (the toner-attaching portion) unnecessary for copying at the photosensitive member is expanded. However, the light is reflected from the portion at the original support plate except for the portion supporting the original during the exposure-scanning, so that when the side edges of photosensitive member are exposed to the light, the charge at the same is dissipated, thereby preventing the unnecessary toner from attaching thereto. The area for the erase light source at the side edges of photosensitive member is changed corresponding to the size of copying paper as above mentioned to thereby ensure that toner is prevented from unnecessarily attaching to the photosensitive member, but when the image of original is reduced unnecessary attachment of toner cannot be, fully prevented. In brief, in a case where the area for the erase light source is changed corresponding to the aforesaid copying size, the charge at the side edges of photosensitive member cannot completely be dissipated because the reduction rate is irrelevant to the size of copying paper.
The image of the original can be exposed over the entire surface of the original support plate, and the photosensitive member is larger enough to be exposed to the entire image of the original on the original support plate, but when the original is copied in reduction, only the area corresponding to the reduction size of the plate is exposed to the reflected light therefrom, so that the side edges of photosensitive member are out of the exposure area so as to be not exposed. Hence, the side edges of photosensitive member, when reduced, are always electrified and attached with toner, whereby it is impossible in such a case to completely dissipate the side edge charge, resulting in unnecessary toner being attached to the side edges of the photosensitive member.
An electrophotographic copying apparatus (disclosed in the U.S. Pat. No. 3,724,940) has been developed for dissipating the charge on the side edges of photoelectric member even when reduced. This copying apparatus, which rotates a shade pivoted to one side of the copying apparatus to change the projection position of a correction light source, is not accurately controllable and causes unnecessary toner is attached to the photosensitive member.